1. Field of Invention
The present invention relates to a circuit for detecting output current information from the winding voltage of a transformer in a Switching Mode Power Supply (SMPS), a circuit for performing feedback to maintain a constant current limit value using the detected output current information even in the case in which the value of a load is equal to or larger than a set value, and an SMPS for limiting output current using primary feedback.
2. Description of Related Art
A power supply circuit for charging the battery of a wireless phone includes a charge current control unit, so that charge current is prevented from exceeding an upper limit value. When an abnormality occurs and charge current abnormally increases beyond the upper limit value, the charge current control unit performs control such that output voltage drops and output current is prevented from exceeding the upper limit value. For this purpose, a general SMPS power supply circuit includes an output current feedback circuit.
FIG. 1 shows the circuit of an SMPS having a function of limiting output current according to the prior art. A serial resistor Ris is installed on a load side, and senses current flowing therethrough. If the value of the current flowing through the serial resistor Ris is equal to or larger than a set value, voltage applied to the serial resistor Ris exceeds reference voltage Vref2. The voltage applied to the serial resistor Ris and the reference voltage Vref2 are input to a comparison amplifier U3, and are compared with each other. The results of the comparison are fed back to an error amplifier 12 on a primary side through an opto-coupler OptoA and an opto-coupler OptoB, are compared with reference voltage Vref1 again, and are amplified. The output of the error amplifier 12 is input as the control input of a control unit 11. When voltage that is higher than the reference voltage Vref2 is applied to the serial resistor Ris, the output of the error amplifier 12 performs control on the control unit 11 such that output voltage drops. Therefore, output current which flows through the load is prevented from exceeding a set value.
The output current feedback circuit of the SMPS having the function of limiting output current according to the prior art of FIG. 1 has a problem of decreased efficiency because power is lost due to the voltage drop in the serial resistor Ris for sensing output current, and a problem of increased cost because, in addition to a main semiconductor circuit on a primary side, relatively expensive components, such as the comparison amplifier U3 and the opto-coupler ‘OptoA’, should also be used on a secondary side.